The calculator's own default courses — an A (3 credits), a B+ (4 credits), and an A- (3 credits) — give a GPA of 3.63. A simple average of the three grade points (4.0, 3.3, 3.7) would give 3.67 instead — close, but not the same number, because the 4-credit B+ pulls the weighted result down more than an unweighted average would let it. The gap between the two methods grows with the size of the mismatch between a course's credit load and the rest — a single-credit seminar with a poor grade barely moves a credit-weighted GPA, while the same grade in a 4-credit core course moves it substantially.
Adding a fourth course to the same example — an F worth 3 credits — drops the GPA from 3.63 to 2.79. That's a bigger drop than it might look like at first, because an F contributes 0.0 grade points while still adding its full credit hours to the denominator: it drags the average down without contributing anything to offset it, unlike simply not taking a course at all (which leaves the GPA at 3.63, untouched). This is also why retaking a failed course, where the institution allows it, tends to matter more for GPA recovery than an equivalent grade in a brand-new course — replacing a 0.0 in the calculation does more work than adding a new positive number next to it.
A standard 4.0 scale, A equals 4.0 down to F equals 0.0. Some schools use different scales, plus/minus grading, or weighted honors and AP courses — check your institution's official policy for exact records.
Each grade's point value is multiplied by that course's credit hours, then all of those are summed and divided by total credit hours — so higher-credit courses count more.
Yes — add every course from every semester as its own row.